June 9, 2011

Diving Bell Spiders Spend Entire Life Underwater

by ssavage

It has 8 hairy legs, spins webs, hunts prey and lives its entire life under the surface of the water. No, it is not the something from a bad horror film, the Argyroneta aquatica, or Diving Bell Spider is the subject of a fascinating study published in the Journal of Experimental Biology.

"It is an iconic animal; I had read about the spider as a small boy in popular literature about ponds," says Roger Seymour from the University of Adelaide. Each spider constructs a net of silk in vegetation beneath the surface and fills it with air carried down on its abdomen. The spiders spend their entire lives submerged and even lay their eggs in their diving bells, BBC News reports.

The Latin genus name Argyroneta means "silvery net" because of the shining appearance of the spiders' bubble webs underwater.

In the lab, Seymour simulated a stagnant, weedy pond on a hot summer day and tested how the spiders fared in the challenging conditions with a device called an optode. Seymour gingerly poked an oxygen sensing optode into the bubble to see how the animal reacted.

The spider did not appear to react, so the team continued recording the oxygen level. "Then it occurred to me that we could use the bubble as a respirometer," says Seymour, to find out how much oxygen the spiders consume.

Measuring the oxygen in the bubble and surrounding water, the research team calculated the amount of oxygen flowing into the bubble before calculating the spider's oxygen consumption rate.

The study found that the diving bell could extract oxygen from the most stagnant water even on a hot day and that the metabolic rate of the aquatic spider was low and similar to the low metabolic rates of other spiders that wait for prey to pass by.

"As the spider consumes oxygen from the air in the bell, it lowers the oxygen concentration inside. The oxygen can decrease below the level of dissolved oxygen in the water, and when this happens, oxygen can be driven into the bubble from the water," explained Seymour.

"The carbon dioxide that the spider produces is not a problem at all, because it is readily dissolved in the water and it never builds up." Unlike animals that exchange oxygen and carbon dioxide across gills however, the spiders have to contend with the other gases in the air they transport.

"If you absorb one gas from a gas mixture in a collapsible bubble, the remaining gases must increase in concentration. Because oxygen is taken from the bubble air, and CO2 does not build up, it causes the nitrogen in the bubble to rise in concentration," he said.

As the nitrogen disperses from the bubble, the bubble collapses but it does so slowly, roughly over the course of a day according to the scientists' results.

"The spider is able to remain in the diving bell on very hot days, when its metabolic rate is higher than normal, if the water is well oxygenate." This means the spiders can return to the surface infrequently, avoiding the risk of being caught by predators such as birds.